Starch is a well-known polysaccharide consisting of a large number of glucose monosaccharide units joined together by glycosidic α(1,4) or α(1,6) bonds. The sources of starch for industry purposes are predominantly agricultural products, e.g. starchy vegetables and cereals such as rice, wheat, maize, cassava, potato etc.
By hydrolysis of starch simpler carbohydrates are obtained, which can be used in the production of e.g. syrup. All poly- and oligosaccharides, which are usually a product of enzymatic and/or acid hydrolysis of starch, are often referred to as dextrins. The product of such a hydrolysis is often described using a DE-value (dextrose equivalent), which is based on the ratio of dextrose to dry-matter. The hydrolysates of starch are available in various grades. Thus at low grade glucose syrup is available with a DE value of 20-25, a normal glucose syrup has a DE value of 43, a maltose syrup has DE value of around 40, and glucose sugar has a DE value of 87-98.
Depending on the source of the starch and the process used for the hydrolysation, a sludge fraction of varying extent may be obtained as part of the product. The state of the art technology for removing said mud fraction from the product is by use of a rotary vacuum filter (RVF) with a diatomaceous earth (kieselguhr) coating. The use of RVF is commonly associated with high investment and high operating costs i.e. high cost for kieselguhr and for disposal of kieselguhr. RVF further introduces a high complexity of the process as well as safety issues with respect to operators. Furthermore, sugar loss in the kieselguhr occurs.
A method for purification using ultrafiltration has been suggested in WO 2005/079945 and U.S. Pat. No. 4,154,623. According to the published international patent application a filtration unit comprising a multiple feed and bleed microfiltration battery and a three-phase-decanter is disclosed. The three-phase-decanter decanter is installed in-between the multiple feed and bleed microfiltration battery and the ultrafiltration unit. Membranes applied in the microfiltration battery are ceramic membranes having filtration layers with a pore size of from 0.050 μm to 0.8 μm. The filtrate from the microfiltration battery is separated in a three-phase decanter into a heavy phase, an intermediate heavy liquid phase and a light phase. The intermediate and the heavy phase is finally treated in an ultrafiltration unit.
According to the U.S. Pat. No. 4,154,623, a starch slurry is liquefied at 85° C. using an α-amylase. Subsequently, the liquefied product is saccharificated using a fungal amylase or an amyloglycosidase at about 60° C. The saccharified product is filtered on a micro filter having a pore size of 0.1-0.3 μm, i.e. through a micropore filter. The filtrate is introduced into an ultrafiltration apparatus to provide a permeate stream, which is further processed to a syrup.
The object of the present invention is to provide a method and an apparatus for preparing a purified liquid of carbohydrates at low cost over long production cycles.